1. Field of the Invention
The invention generally relates to wireless communication devices, and more particularly, to mixer circuits on integrated circuits that are used for converting between radio frequency (RF) signals and baseband signals in wireless communication devices.
2. Description of the Related Art
The increasing usage of wireless communication devices like cellular phones and other types of wireless communication devices is based on significantly improved technologies allowing to provide integrated circuits implementing the electronic necessary for wireless communication at a smaller size and, at the same time, at a reduced price. A key component of a wireless communication device is a mixer circuit. The mixer circuit may be operated for up-conversion allowing to convert an input frequency to the radio frequency (RF) or for down-conversion allowing to convert the frequency between the radio frequency (RF) received by a wireless communication device and baseband signals or intermediate frequency (IF) signals to be further processed by the wireless communication device. The down-conversion of RF signals is crucial for the extraction of information carried on a desired RF signal among all the other information carried by other RF signals.
Although the super-heterodyne receiver is still the most common architecture, the direct conversion receiver has gained much attention in recent years as a possible solution for a single-chip receiver as the direct conversion receiver does not require any other off-chip structures in the signal path. Direct conversion offers a lot of advantages over its predecessor technology, the super-heterodyne architecture. It eliminates the need for many components, resulting in wireless communication devices with reduced cost and size. In particular the entire intermediate frequency subsection is eliminated.
Although the direct conversion architecture does not require the use of external filters, there is still a number of problems to be solved, among which the DC offset generated and the flicker noise are the most critical.
The frequency translation in a direct conversion receiver or a super-heterodyne receiver is performed by a mixer. Mixer topologies are classified as active or passive according to the mixer's ability to provide gain or not. Both types can be realized in CMOS technology. Active mixers achieve conversion gain and require reduced local oscillator (LO) power. The primary advantage of passive mixers is increased dynamic range at the expense of LO power.
It is further to be differentiated between single balanced mixers and double balanced mixers. Single balanced mixers reject one of the input or LO signals at the output, while double balanced mixers reject both. Single and double balanced mixers are both used in practice. A particular double balanced structure which is popular for use in integrated receivers is known as Gilbert cell mixer.
In most configurations, a mixer performs a frequency translation by commutating a current signal with a switching differential pair driven by a strong LO voltage. The principle of operation is the same for bipolar and CMOS technologies.
An exemplary embodiment of a wireless communication device is shown in FIG. 1. The communication device 1 is connected to a station 2 via a wireless communication link 3. The communication device 1 is designed for transmitting and receiving signals via antenna 4. A communication signal transmitted from station 2 to the communication device 1 is received by antenna 4 and propagated to low a noise amplifier (LNA) 5 for amplifying the received radio signal. A down-converter mixer 6 converts the radio frequency to a lower frequency. The down-conversion is performed in the mixer 6 by multiplying the received RF signal with a local oscillator (LO) signal provided by LO signal generator 7. In the reverse direction, a low frequency input signal is received by up-converter 8 and converted to a radio frequency (RF). The up-conversion is carried out by up-converter mixer 8 multiplying the input signal and the LO signal provided by LO signal generator 9. The output RF signal is applied to power amplifier (PA) 10 and transmitted via antenna 4 to station 2.
Referring to FIG. 2, the basic principle of a mixer as used for instance in the above-mentioned up-converter 8 and down-converter 6 is shown. Mixer 11 receives two signal inputs, namely a first input signal 12 having a frequency F1 and a second input signal 13 having a frequency F2. The mixer 11 transforms these frequencies by multiplication into an output signal 14. The output signal comprises a differential frequency (either F1−F2 or F2−F1) and a sum frequency (F1+F2) of the input frequencies F1 and F2.
In a transmitter as described for instance in FIG. 1 the baseband frequency is transformed into a radio frequency. For this purpose, the sum frequency of the mixer is used in the up-conversion mixer. In contrast, a receiver transforms the received radio frequency into the baseband signal. For this purpose, the difference frequency supplied by the mixer is used.
The unwanted signal, the image frequency, may be incident on a converter mixer. The unwanted signal at the image frequency may be rejected by an image rejection mixer, the principle of which is shown in FIG. 3. Such an image rejection mixer is intended for the conversion of signals having signal components in quadrature. When both signals are mixed with an LO signal they have a 180 degrees delay at the output between the image and the wanted signal. A simple subtraction at the output generates the rejection of the image.
An example of a double balanced active mixer is shown in FIG. 4. The mixer illustrated in FIG. 4 is commonly known as a Gilbert cell mixer. The Gilbert cell mixer comprises a mixer core including four transistors Q1, Q2, Q3, Q4 and a RF input section including transistors Q5 and Q6. The transistors of the mixer core Q1, Q2, Q3 and Q4 are differentially connected in a common emitter configuration and in turn connected to a differentially connected pair of transistors Q5 and Q6.
As shown in FIG. 4, the Gilbert cell mixer has two input ports 16, 17 and one output port 18. The LO signal is applied to the LO input port 17 and, when used in a down-converter, the RF input signal is applied to the RF input port 16. The resulting IF signal is provided at the intermediate (IF) output port 18. Further, the mixer circuit comprises a supply voltage input 19.
The RF input port 16 receives two RF signals from an antenna, while the LO input port 17 receives two LO signals provided by a LO circuit. Such an LO circuit is included with a wireless communication device as shown for instance in FIG. 1. The two RF signals are inverted (i.e. 180 degrees out of phase) with respect to each other, as are the two LO signals. The double balanced mixer 15 multiplies the RF signal and the LO signal provided at the RF and LO input ports 16, 17, respectively, to produce first and second intermediate signals at the IF output port 18.
Such mixers have still a number of problems. One problem is that Gilbert cell mixers produce spurious signals due to transistor size miss-match and other circuit imperfections. These spurious signals degrade the performance of a direct-conversion receiver.
Another problem of Gilbert cell mixers is flicker noise from the mixer commutating switches, especially in CMOS implementations. MOS transistor flicker noise degrades the mixer noise performance for low output frequencies that are exploited in a direct conversion receiver. Flicker noise does not only degrade the noise performance of these mixers, but also adds noise directly to the base band.
An additional well-known problem of the Gilbert cell mixer architecture are DC offsets.
In view of these drawbacks of the prior art it is therefore the primary object of the invention to provide an improved mixer circuit, an improved direct conversion receiver and an improved method for operating a mixer circuit.